Method of treating parts for kitchen utensils

ABSTRACT

A method for processing parts for kitchen tools in order to protect the parts from scratches, includes sequentially: a nitridation step, optionally including a nitrocarburizing step, between 592 and 750° C. in order to promote the formation of a nitrogen austenite layer; and a processing step for promoting the conversion of at least a portion of the nitrogen austenite into a phase with reinforced hardness.

The invention concerns a method of treating parts for kitchen utensilsof ferrous alloys that are non-stick, scratch resistant and corrosionresistant and parts treated by the method.

It is known to use different materials, or stacking of materials, toproduce kitchen utensils: steel (alloyed or not), aluminum, stainlesssteel (that is to say, generally containing more than 11% chromium),copper or silver alloys in particular, with or without surface coatingssuch as polymer layers based on polytetrafluoroethylene (PTFE,distributed in particular under the trademark Teflon). Each material hasits own advantages and drawbacks for this type of application.

Aluminum is very resistant to corrosion during steps of washing theutensils, including in a dishwasher with detergents, but on the otherhand can easily be scratched and its non-stick properties are mediocre.For this reason it is often associated with a coating ofpolytetrafluoroethylene type.

Austenitic stainless steel (containing approximately 18% chromium and10% nickel) also has good corrosion resistance and slightly betterscratch resistance than aluminum. On the other hand, it is a poor heatconductor which does not facilitate the homogenization of temperaturefor cooking utensils such as woks, frying pans, griddles, casseroledishes, pots, broiler plates, fryers, grills (barbecues), molds orsaucepans.

Copper is a very good heat conductor which is recognized for providinggood quality cooking. However, it is an expensive material reserved fortop of the range utensils.

Non-stainless steels have a great advantage over all the otheraforementioned materials, which is their price. To be precise, steels,especially unalloyed steels (those without any added component) orweakly alloyed steels (that is to say in which no added componentexceeds 5% by weight), are easily and abundantly available, their priceis low and varies little relative to that of stainless steels or copper.It is for this reason that non-stainless steels are very widely used asthe basic material for bottom of the range utensils.

However, these steels have very low corrosion resistance, especially oncleaning the utensils with detergents (washing in a dishwasher is ruledout), their surface is easily scratched and their non-stick propertiesare mediocre.

The teaching of patent US 2008/0118763 A1 shows that ferriticnitrocarburizing may be applied to kitchen utensils at a temperature of1060° F. (571° C.) for 3 h in an atmosphere of 55% nitrogen, 41% ammoniaand 4% CO₂. Carried out next are gaseous oxidation (post-oxidation) at atemperature of 800° F. (˜427 ° C.) and temporary protection by baking at500 ° F. (260° C.) for 45 minutes using a cooking oil. According to thisdocument, the treated surfaces have increased hardness and improvedcorrosion resistance.

The treatments of nitriding, nitrocarburizing, oxinitriding andoxinitrocarburizing (the prefix oxi- means that after the nitriding ornitrocarburizing, an oxidizing step is carried out) are used in themechanical industry (in the automotive sector: valves, gas struts, balljoints; in construction equipment: articulations, hydraulic jacks, etc.)

These treatments are carried out industrially either using gas(ammonia-based atmospheres) or using plasma (glow discharge at lowpressure), or using liquid (ionic liquid media, see for example thedocument US2003084963).

In industry, the treatments of nitriding, nitrocarburizing, oxinitridingand oxinitrocarburizing are conventionally carried out in the ferriticphase (in the iron-nitrogen diagram), that is to say at temperaturesless than 592° C.

A layer of iron nitride is formed, and the layer below is referred to asdiffusion layer.

Beyond 592° C., the γN phase forms (nitrogen-containing austenite,generally named γN) between the nitride layer and the diffusion layer.Nitrogen-containing austenite is a microstructure that is particular tosteel. The precise temperature beyond which the γN phase forms dependson the exact composition of the steel. If the latter contains a lot ofalloying components, this temperature limit value may shift up to 600°C.

This nitrogen-containing austenite layer transforms intonitrogen-containing braunite, another microstructure particular tosteel, under the effect of temperature at the oxidation step which isconventionally carried out after the nitriding or nitrocarburizing step.However, in the field of mechanical parts, the oxidation step isgenerally carried out since it is desired that the parts be corrosionresistant, nitriding increasing wear resistance and the oxidationincreasing the corrosion resistance.

This retransformation into braunite is not generally desired since forthe mechanical applications for which nitrocarburizing is generallydestined, the presence of a nitrogen-containing braunite layer givesrise to fragility in case of impact.

More particularly, the typical mechanical stresses whose effect it isusually sought to limit by nitrocarburizing are cyclic stresses and/oralternating stresses which will recur with a high number of cycles, suchas for example superficial fatigue or impact.

The presence of a braunite layer is thus generally ruled out since thefragility of that layer may lead to flaking or splitting of the nitridelayer under the effect of impact (high energy transfer which is briefand localized between two parts moving relative to each other).Nitrocarburizing and nitriding are thus conventionally carried out inferritic phase. When austenitic nitriding is carried out, thepost-oxidation step is then generally performed at a temperature below200° C. to avoid the retransformation of the nitrogen-containingaustenite into braunite (see for example patent EP1180552).

Furthermore, with regard to the teaching of patent US 2008/0118763 A1,the applicant has noted that at the post-oxidation step carried out justafter the nitrocarburizing, the high temperatures used (greater than200° C.) give rise to tempering in the diffusion zone. The consequenceof this annealing is a drop in the hardness of the diffusion zone whichis detrimental to the scratch resistance of the kitchen utensilstreated.

Consequently, when a load is applied which stresses the material in itscore and not just the hard surface layer, the substrate deforms and thehard surface layer splits and flakes.

The same applies to the step of baking the temporary protective agentwhich is carried out between 150 and 260° C., as well as during the lifeof the utensils, at each utilization above 200° C. of those kitchenutensils.

This is in particular detrimental in the case of the low carbon steelswhich are generally used for kitchen utensils.

It is moreover to be noted that nitrocarburizing methods require a highenergy input, and that it is desirable to control the treatment time, tolimit the final costs. One of the drawbacks of the treatment rangepresented by document US 2008/0118763 A1 is its duration which is long(3 hours).

In this context, the problem which the invention sets out to solve is togive improved non-stick, scratch resistant and corrosion resistantproperties to the surface of kitchen utensils made of steel (not alloyedor weakly alloyed), with improved production costs.

To solve this problem a method is provided of treating parts for kitchenutensils characterized in that it comprises successively:

-   -   a nitriding step between 592 and 750° C. so as to promote the        creation of a nitrogen-containing austenite layer    -   a treating step adapted to promote the conversion of at least        part of the nitrogen-containing austenite into a phase of        enhanced hardness.

The method is remarkable in that it is implemented to protect the partsfor kitchen utensils against scratches.

The initial hardening of the parts (nitriding step) may be carried outeither by austenitic nitriding, or by austenitic nitrocarburizing. Itshould be clearly understood that what is meant by nitrocarburizing is atreatment by diffusion of nitrogen and carbon, considered as aparticular case of nitriding, by which term a treatment is designated inthe general sense involving at least a diffusion of nitrogen. Theaustenite layer created is buried under the nitride layer, above thediffusion layer.

The subsequent treating step, which may in particular be a heattreatment or a thermo-chemical treatment, results in enhancing thehardness of the nitrogen-containing austenite, the nature of whichchanges. The hardness is measured using the standard protocols. By wayof example, it is preferably enhanced by at least 200 HV_(0.05) orpossibly 300 HV_(0.05).

According to a first embodiment, the phase with enhanced hardness isbraunite. The conversion may in this case in particular be carried outby passing to over 200° C. for a time longer than 10 minutes. In anexample relative to this embodiment, the hardness of the phase thatchanges nature thus passes from approximately 400 HV_(0.05) toapproximately 800 HV_(0.05).

The treating step is adapted to enable the conversion of thenitrogen-containing austenite layer into nitrogen-containing braunite.For this, in particular, it is carried out with a low content ofactivated nitrogen around the parts. By activated nitrogen is meant,depending on the nitriding route used, gaseous ammonia, ionized nitrogenor molten nitrogenous salts.

A simple way to implement the conversion is to eliminate any presence ofactivated nitrogen in the medium in which the parts are placed, but itis possible merely to reduce the concentration of those activatedspecies sufficiently to stop the nitriding reaction. The conversion isimplemented at a temperature less than or equal to the nitridingtemperature, for example at a temperature less than 480° C.

It is to be noted that between the nitriding step and the conversionstep, the parts may be moved, or be kept in the same place.

Furthermore, the conversion step may be carried out just after thenitriding step, without the parts having been cooled, which enablesfavorable kinetics to be obtained, but it may also be carried out aftera lapse of time during which the parts have been subjected to ambienttemperature.

According to a second embodiment, the phase with enhanced hardness isnitrogen-containing martensite, and the conversion may in particular becarried out by a passage to below −40° C. for a time longer than 5minutes. Nitrogen-containing martensite is a particular microstructureof steel, different from nitrogen-containing austenite and frombraunite. In an example relative to this embodiment, the hardness of thephase that changes nature thus passes from approximately 400 HV_(0.05)to approximately 750 HV_(0.05).

For the application for cooking utensils, the applicant has found thatthe stacking of layers of materials so obtained with the method hasbetter resistance to scratches made by pointed utensils (forks, knives)than a stacking obtained by ferritic nitriding. It would seem that thebraunite or martensite layer formed during the conversion step serves asa support for the nitride layer situated below.

More particularly it would appear that at the time of surface mechanicalstresses typical of a kitchen utensil use (stirring, cutting of food),the area of contact between the cooking utensils and the pointedutensils is very small).

With nitriding or ferritic nitrocarburizing, the applicant has found, asmentioned above, that the nitride layer collapses locally since thediffusion layer is not sufficiently hard (200-250 HV_(0.05) for lowcarbon non-alloy steels) to support it. Localized deformation of thepart occurs, as well as of the nitride layer which splits and flakes.

Without wishing to be bound by a particular explanation, it would seemthat with austenitic nitrocarburization, the nitrogen-containingaustenite retransformed into braunite or into martensite providesmechanical support for the nitride layer having much better performancethan that given by the diffusion layer alone in the parts not havingbeen treated according to the invention. The nitride layer no longerdeforms under the typical mechanical stresses of cooking utensils, whicheliminates the scratching phenomena.

The same applies for corrosion resistance. By nature, the nitride andoxide layers are passive layers, that is to say they do not rust.Corrosion of oxinitrided or oxinitrocarburized parts may however occursince the nitride and oxide layers are never free from defects. Theelectrolyte may then enter into contact with the substrate whichconsequently corrodes.

Limiting the risk of scratches to the nitride and oxide layers by virtueof the treatment according to the invention protects against thecorrosion of the cooking utensils treated according to the invention.

It is noted that the effect observed is linked to the application forthe kitchen utensils, for which the frequency of stressing of thesurface is low (a few jabs with a knife or spatula from time to time)and not generally at the same location (it is rare for several tens orhundreds of jabs by a knife to be given exactly at the same location ina frying pan). The method thus applies advantageously to utensils suchas woks, frying pans, griddles, casserole dishes, pots, broiler plates,fryers, grills (barbecues), molds or saucepans, and in particular totheir surfaces destined to enter into contact with food during cooking.The utensils are adapted to be used for domestic, group, restaurant orindustrial cooking for the preparation of cooked food destined forexample to be packaged and distributed.

It thus seems that the advantageous character of the presence of thebraunite or martensite layer is due to the fact that it makes itpossible to avoid excessively steep hardness gradients (as is the casebetween the nitride layer and the diffusion layer with conventionalnitriding of steels of XC10-XC20 type).

The braunite or martensite layer, which is of hardness intermediatebetween that of the nitride layer and that of the diffusion layerapparently reduces this gradient in such a manner that better mechanicaldurability is obtained. This is all the more advantageous in that, aswas mentioned above, the oxidation step leads to a drop in hardness inthe diffusion zone.

Furthermore, using treatment temperatures comprised between 595 and 700°C. it is possible to multiply by two or three the diffusion kineticsrelative to a treatment carried out between 530 and 590° C., whichenables the treatment cost to be reduced and to reduce the energy needsrequired to perform it.

In certain advantageous modes of implementation, the treating stepadapted to promote the conversion into braunite is also a controlledoxidation step, which in addition enables an enhanced protective effectagainst corrosion to be obtained.

Alternately, or in combination, the conversion into braunite comprisesbaking at more than 250° C. for a time comprised between 20 minutes and3 hours and this baking is further to or precedes an oxidation inboiling brine between 120 and 160° C. The brine may in particular be ata temperature between 130 and 145° C.

According to one procedure for implementation, the method, whichinvolves a conversion into braunite or martensite, further comprisesoxidation using gas between 350 and 550° C.

Alternatively or in combination, it comprises an oxidation by baths ofmolten salt between 350 and 500° C.

Alternatively, or in combination, it comprises an oxidation by boilingbrine between 120 and 160° C., or between 130 and 145° C.

Preferably, the nitriding comprises a nitrocarburizing phase. It mayalso comprise a phase of nitriding alone followed by or preceded by anitrocarburizing phase. Thus, it is possible for the nitrocarburizingphase to be complemented by a phase of nitrogen diffusion without carbondiffusion.

The nitrocarburizing is advantageous since it makes it possible toobtain single phase nitride layers to be obtained which improves themechanical durability of the parts, in particular to impacts orscratches, for example, beyond what is obtained when the invention isimplemented with nitriding without nitrocarburizing.

According to one embodiment, the nitriding comprises nitriding ingaseous phase which may comprise nitrocarburizing in gaseous phase.According to another embodiment, it comprises nitriding with plasmawhich may comprise nitrocarburizing with plasma.

According to a third embodiment, it comprises nitriding in an ionicliquid medium which may comprise nitrocarburizing in an ionic liquidmedium.

According to an advantageous feature, the nitriding is carried out for atime comprised between 10 minutes and 3 hours, and preferably between 10minutes and 1 hour.

It may preferably be carried out at a temperature comprised between 610and 650° C.

The method is advantageously complemented by prior degreasing of theparts.

Furthermore the method advantageously comprises a step of prior heatingof the parts to treat between 200 and 450° C. in an oven for a timecomprised between 15 and 45 minutes, after the degreasing and before thenitriding, so as to prepare the parts for the nitriding. This enablestime to be saved in the implementation of the method, in particularbecause the parts do not cool the reaction medium when they areintroduced therein.

According to another advantageous feature, the parts receive temporaryoily protection at the end of the treatment, to increase their corrosionresistance, beyond the protective effect already obtained with thetreatment according to the invention without that additional protection.

Finally, the method is advantageous in that in addition it gives thetreated parts wear resistance properties and non-stick properties.

It is to be noted that the method is in particular applied to parts offerrous alloy comprising at least 80% of iron by weight, or even tonon-alloy or weakly alloyed parts.

The invention also provides kitchen utensils treated by the methodaccording to the invention.

The invention will now be described in detail, with reference to theaccompanying drawings, in particular.

FIG. 1 which shows a hardness profile measured on a similar cookingutensil treated by a method of the prior art,

FIG. 2 which shows a hardness profile measured on a cooking utensiltreated according to a preferred embodiment of the invention,

FIG. 3 which presents a superposition of the two preceding profiles.

The treatment range may be broken down into several steps: First of all,degreasing of the parts is carried out to eliminate any trace of organiccompounds on the surface which could hinder the diffusion of nitrogenand/or carbon.

Next, the parts are brought to austenitic nitrocarburizing or nitridingtemperature (between 592 and 750° C.), but preferably to temperaturescomprised between 610 and 650° C. The nitriding or nitrocarburizingtreatment is of a duration comprised between 10 minutes and 3 hours,preferably from 10 minutes to 1 hour.

In a third phase, the parts are oxidized at a temperature comprisedbetween 350 and 550° C., preferably from 410 to 440° C.;

Alternatively, oxidation at a temperature between 120 and 160° C. inboiling brine may be carried out, preferably between 130 and 145° C.

In this case, baking of the parts at a temperature greater than 250° fora time comprised between 20 minutes and 3 hours, preferably 1 hour, isnecessary to convert the γN layer into braunite.

The parts lastly receive temporary protection in the form of afood-grade oil to increase their corrosion resistance, beyond the effectof protection already obtained with the treatment according to theinvention without that additional protection.

Tests have shown the great advantages obtained by the range of treatmentas provided by the invention. Austenitic nitrocarburizing was carriedout at 640° C. for 45 minutes in an ionic liquid medium containing 15%cyanates, 1% cyanides and 40% carbonates by weight.

The parts were then directly tempered in an oxidation bath at 430° C.for 15 minutes. Next, the parts were cooled in water, rinsed and dried.At the end, the food-grade oil (sunflower oil) was applied to thesurface to increase the corrosion resistance.

The morphology of the oxide layer serves as a sponge for the film of oilthat remains trapped in the microporosity of the layer. Although it isnot necessary to carry out a final baking step, this may be carried outin order to promote the retention of the oil by the oxide layer.

The treatment results in greatly increasing the hardness of the layersupporting the nitride layer, relative to a treatment according to theprior art.

FIG. 1 shows the hardness profile (measured using the Vickers standardprotocol), for a part (steel XC10) treated according to the prior art(ferritic nitrocarburizing and oxidation). The hardness is measured on across-section. The hardness of the nitride layer 100 is of the order of1000 HV_(0.05), whereas the hardness of the diffusion layer 110 is ofthe order of 180 HV_(0.05). The transition between the hardnesses of thetwo layers is abrupt, over less than 3 microns, at a depth of in theneighborhood of 20 microns.

FIG. 2 shows the hardness profile for an identical part, treatedaccording to the described embodiment of the invention. The hardness isalso measured on a cross-section. The hardness of the nitride layer isof the order of 1000 HV₀₀₅, and that of the diffusion layer of the orderof 180 HV_(0.05). Two transitions are visible in the hardness profile:one at 20 microns, and the other at 28 microns. The hardness of theintermediate layer, referred to as nitrogen-containing braunite layer isof the order of 820 HV_(0.05). The overall gradient is smaller than inFIG. 1.

FIG. 3 shows the comparison between the hardness profiles observed afterthe treatment according to the invention, and after the treatment offerritic nitrocarburizing and oxidation.

The hardness of the intermediate layer 205 is comprised between that ofthe diffusion layer 210 and that of the nitride layer 200.

Furthermore, the range thus produced takes only one hour of temperature,which clearly shows the efficiency of the invention in energy terms.

The utensils obtained have enhanced non-stick properties, shown by theease of cleaning of burnt food after use.

Alternatives to the treatment presented will now be detailed. Thenitrocarburizing treatment may be carried out in gaseous phase withatmospheres based on ammonia (NH₃), nitrogen (N₂) and one or morecarbon-containing gases such as methane, ethane, propane, butane,pentane, acetylene, carbon monoxide, carbon dioxide, endothermic gas,and exothermic gas.

The nitrocarburizing treatment may also be carried out using plasma: ina vessel under reduced pressure (typically 5-7 mbar) the parts arepolarized under high voltage. A glow discharge is then created and thegas mixture (typically 79.5% N₂+20% H₂+0.5% CH₄) is dissociated whichenables the activated carbon and nitrogen to diffuse.

The nitrocarburizing treatment may also be carried out using liquid(ionic liquid media), as mentioned, in a bath of molten carbonates,cyanates and cyanides. The cyanate ions (CNO⁻) serve as a source ofnitrogen whereas the traces of cyanides (CN⁻) serve as a source ofcarbon.

The oxidation step must be controlled and may be carried out using gaswith oxidizing atmospheres such as air, controlled N₂/O₂ mixtures,steam, nitrous oxide, etc. In all cases the aim is to form, attemperatures comprised between 350 and 550° C., a layer of black ironoxide Fe₃O₄, which is a passive oxide which, once formed, avoids theformation of rust iron oxide Fe₂O₃ which is red).

The oxidation may also be carried out in ionic liquid media attemperatures comprised between 380 and 470° C., for times ranging from 5to 40 minutes.

The oxidation may lastly be carried out in brine (mixture of water,nitrates, hydroxides) at a temperature comprised between 100 and 160°C., for times ranging from 5 to 40 minutes.

In this case, post-heat-treatment at a temperature greater than 250° C.is necessary to retransform the layer of γN into braunite.

According to a second embodiment, the nitrogen-containing austenite isre-transformed into nitrogen-containing martensite by cryogenictreatment between −40 and −200° C. for a time comprised between 5minutes and 3 hours, preferably between 1 hour and 2 hours.

The nitrogen-containing martensite is a structure whose hardness is inthe neighborhood of that of the nitrogen-containing braunite. Theapplicant has found that the effect of mechanical support for the ironnitride layer is provided.

According to this embodiment, the treatment range is then the following:

-   -   de-greasing to remove any trace of organic product    -   preheating to a temperature comprised between 250 and 400° C.,    -   austenitic nitrocarburizing between 592 and 650° C.    -   cooling to ambient temperature    -   cryogenic treatment at a temperature between −40 and −200° C.,    -   oxidation either using gas, or by salt baths, or in boiling        brine.

In this embodiment, the applicant has found that the oxidation by aboiling brine is advantageous since it enables hardness of thenitrogen-containing martensite to be obtained that is greater by 100Vickers than that obtained with oxidation at high temperature (more than300° C. using gas in particular).

The invention is not limited to the described embodiments, butencompasses all the embodiments within the capability of the personskilled in the art.

1. A method of treating parts for kitchen utensils for protecting saidparts against scratches characterized in that it successively comprisesa nitriding step, which may comprise nitrocarburizing, between 592 and750° C. so as to promote the creation of a nitrogen-containing austenitelayer a treating step adapted to promote the conversion of at least partof the nitrogen-containing austenite into a phase of enhanced hardness.2. A method according to claim 1 characterized in that the phase withenhanced hardness is nitrogen-containing braunite.
 3. A method accordingto claim 2 characterized in that the conversion is carried out at morethan 200° C. for a time longer than 10 minutes.
 4. A method according toclaim 1, characterized in that the treating step adapted to promote theconversion is also a step of controlled oxidizing.
 5. A method accordingto claim 1 characterized in that the phase with enhanced hardness isnitrogen-containing austenite.
 6. A method according to claim 5characterized in that the conversion is carried out at less than −40° C.for a time longer than 5 minutes.
 7. A method according to claim 1characterized in that the method further comprises oxidation by moltensalt bath, between 350 and 500° C.
 8. A method according to claim 1characterized in that the method further comprises oxidation using gas,between 350 and 550° C.
 9. A method according to claim 1 characterizedin that the method further comprises oxidation in boiling brine, between120 and 160° C.
 10. A method according to claim 1 characterized in thatthe nitriding comprises a nitrocarburizing phase, which may becomplemented by a phase of nitrogen diffusion without carbon diffusion.11. A method according to claim 1 characterized in that the nitridingcomprises nitriding in an ionic liquid medium which may comprisenitrocarburizing in an ionic liquid medium.
 12. A method according toclaim 1 characterized in that the nitriding comprises nitriding withplasma which may comprise nitrocarburizing with plasma.
 13. A methodaccording to claim 1 characterized in that the nitriding comprisesnitriding in gaseous phase which may comprise nitrocarburizing ingaseous phase.
 14. A method according to claim 1 characterized in thatthe nitriding is carried out for a time comprised between 10 minutes and3 hours, and preferably between 10 minutes and 1 hour.
 15. A methodaccording to claim 1 characterized in that the nitriding is carried outat a temperature comprised between 610 and 650° C.
 16. A methodaccording to claim 1, characterized in that prior degreasing of theparts is carried out.
 17. A method according to claim 1, characterizedin that it further comprises a step of prior heating of the parts totreat between 200 and 450° C. in an oven for a time comprised between 15and 45 minutes.
 18. A method according to claim 1 characterized in thatthe parts receive temporary oily protection at the end of the treatment.19. A method according to claim 1, characterized in that in addition itgives the treated parts wear resistance properties and non-stickproperties.
 20. A method according to claim 1, characterized in that itis applied to parts of ferrous alloy comprising at least 80% of iron byweight, and for example to non-stainless steel parts.
 21. A methodaccording to claim 1, characterized in that said enhanced hardness isintermediate between that of a nitride layer and that of a diffusionlayer.
 22. Kitchen utensils treated by a method according to claim 1.